1. Field of the Invention
The invention generally relates to an image analysis method, and particularly relates to an analysis method for the evaluation of herniation classification and geometry in the diagnosis of a herniated inter-vertebral disc (HIVD).
2. Related Art
The diagnosis of a lumbar herniated inter-vertebral disc (HIVD) is mainly based on the location at which the herniated disc substances press or break through the ligaments to compress the spinal cord and roots, which results in two categories. First, lumbar HIVD can be classified as central, lateral or both depending on whether the herniated disc substances compress the central spinal cord, one lateral root or both the spinal cord and the two roots. Second, lumbar HIVD can be classified according to the degree to which the disc substances protrude through the ligaments. The first classification, called bulging, is the mildest herniation in which the herniated disc substances do not break through any ligament and the herniated ligaments may therefore touch the spinal cord or roots. In the second classification called protrusion, the disc substances break the inner ligaments so that the herniated ligaments may compress the spinal cord or roots. In the third classification called extrusion, the substances break all except the outermost ligament that only provides low restraint on the substances so that the substances heavily compress the spinal cord or roots. In the most severe classification, called separation (also called free or sequestered disc fragment), the disc substances break all of the ligaments, may extrude past the broken ligaments and may themselves be broken to form a separated structure. Then, the herniated and especially the separated structure can press heavily on the spinal cord or roots. Because the restraints provided by the ligaments impart geometry to the herniated substances, the classification of an HIVD can be judged by analyzing the shape of the herniation.
FIGS. 1(A) and (B) show ideal spatial models in the prior art, illustrating the spatial relations of disc spaces (green areas), vertebral bones (gray areas), and the neighboring spinal cord and roots (red areas). FIG. 1(A) shows the relations between a disc space, the neighboring spinal cord and roots in which the two roots leave from the cord at a position a little above the top border of the disc space. For simplicity, the curved vertebral bones and the spinal cord are represented as having no curvature. The disc-like boundary of disc substances on a transverse section is considered as a circle or an ellipse (because of the elastic restraint of ligaments) and has the highest radius at the middle of the disc space. The spinal root separates from the cord outward and downward (with an angle of about 45-60 degrees). Therefore, we can simply consider that the angular position of the root at every transverse section increases linearly along the direction of gravity. This allows us to calculate an approximately normal angular position (if no herniation) of a root corresponding to the height of the section.
Actually, parts of the boundary of the disc substances on a transverse section may be no longer circle-like because of the appearance of concave or convex features. As FIG. 1 (B) shows, the herniated disc substances form convex features on the boundary, and spurs of the vertebral bone, dural-fat and other substances may obscure disc substances to form concave features on the disc boundary. If the posture of the transverse section is not perpendicular about the spinal axis, the section may cause some bone substances to form concave features at the boundary or an interior bone area of the disc substances. The concave and convex features should be recognized as different from normal parts of the disc, boundary because of their different anatomic meanings. In traditional (boundary representation or constructive solid geometry) solid models, a recognized feature is based on the results of intersection computation between original boundary edges and the lines connecting vertices.
Lumbar HIVD is not accurately diagnosed on the basis of either clinical findings (i.e. patients' history taking and physical examination) or common imaging modalities including X-ray films, computed tomography (CT) and magnetic resonance imaging (MRI). Some imaging techniques that enhance specific anatomic structures can improve the diagnostic rate. For example, discography-enhanced CT (disco-CT) and CT combined with myelography can achieve a higher diagnostic rate than traditional CT (90-72%). However, these enhancing techniques are invasive procedures with higher risk and danger to the human body. The relatively low diagnostic rate on CT or MRI is generally caused by poor resolution on annular ligaments that surround disc substances in the disc space.
The surgical modalities for lumbar HIVD include microdisectomy, percutaneous disectomy (PLD), and traditional disectomy (through partial or total laminectomy). Which modality is used depends not only on the HIVD classification (e.g. microdisectomy can be only applied to the bulging and protruding classifications), but also on the familiarity of surgeons and, more importantly, on the intention and profession of the patient. Because microdisectomy and traditional disectomy are open procedures, accurate shape and position information of the herniation is important for determining from where and how large bone areas should be opened to achieve minimum invasion.